(1) Field of the Invention:
The present invention relates to a gas filter particularly suitable for the filtration of dry gases used in the manufacture of semi-conductor devices.
(2) Description of the Prior Art:
As well known, various gaseous fluids are used in the production of semi-conductor devices. As recently seen in LSI or VLSI technology, integrated circuits have come to be even more highly integrated and also the pattern sizes have become smaller and smaller. As a consequence, the requirments for the treatment of the gaseous fluids used for etching have been made more stringent, e.g., the fluids must be treated, such as by filtering, so that particulates having diameters as small as 0.01.mu. are removed whereas the prior standard was the removal of particulates as small as 0.05.mu..
Heretofore, various filters have been used for filtration of such gaseous fluids, such filters including a filter having glass fibers shaped in a sheet-like or cylindrical form, and a filter having membrane films which are of polyfluorinated olefins such as those sold under the tradename teflon, are laminated in a disc-like form, or are formed into an element by a pleating operation. Since the filter containing glass fibers generally has a non-uniform hole or pore diameter, the filter is unsuitable for use as a filter for assuredly removing particles having particle sizes larger than a specific value. Furthermore, if a binder is mixed into the glass fibers when the glass fibers are formed into a sheet-like or cylindrical shape, the binder may react with various gases. Thus, binder cannot be used in such a filter. As a result, the glass fibers are likely to change positive relative to each other such that once captured particles may be released. In addition, there is the problem that metal components such as Na.sup.+, B.sup.+, etc., contained in the glass fibers may be drawn out by the gases and become a contaminating source for the semi-conductor devices.
In the filters using the membrane films, static electricity is likely to be produced in the membrane films, and thus foreign matter becomes attached to the primary side of the filter due to this static electricity. Although in one aspect, this exhibits a positive effect upon the filtration, the foreign matter is likely to attach to the primary side of the filter due to the static electricity even during the production of the filter. Since this foreign matter is not easily removed by means of clean air or the like, the matter remains attached to the produced filter. After the filter has been used for a long period of time, the foreign matter peels off from the secondary side to unfavorably cause the production of defective products. For example, even if a filter is manufactured in a clean room of Class 100 in which the number of particles of about 0.5.mu. or less is not more than about 100/ft.sup.3 , there is still the possibility that foreign matter of 0.5.mu. is attached to the inside of the filter. In addition, with respect to the disc-shaped filter as well as the element-type filter obtained through the pleating operation, since there are extremely large trapped gas spaces, for instance, spaces between the discs, or in pleat-bent spaces, it takes an extremely long period to completely remove the foreign matter. Furthermore, although the Teflon-type membrane films of 0.2.mu. or more are currently commercially available, a membrane film having pores of 0.1.mu. or less is necessary in order to reduce the amount of foreign matter to present day standards. However, to produce a membrane film with such characteristics involves extremely large start-up costs. Moreover, the production of disc-type or pleat-type filters also involves extremely large start-up costs.
Since the degree of filtering provided by a gas filter has a predominant influence upon the fraction of defective semi-conductor devices produced, the enhancement of the performance of the gas filter is extremely desirable. However, as mentioned above, conventional gas filters have been able to meet the above demands. As shown in FIG. 4, there is a known filter in which a fine ceramic film 3 is formed on the inner surfaces of flow holes 2 of a core 1 shaped in the form of a ceramic honeycomb structural body. In use, liquid A is introduced from the upstream side (left-hand side) to the flow holes 2, and a filtrate is taken out from the outer peripheral surface of the core 1 through the ceramic film 3 while a pressure difference is created by appropriately throttling a pipe line on the downstream side by means of a valve 4. Foreign matter attached to the inner surfaces of the flow holes is cleaned off by the energy of the liquid flowing from the upstream side to the downstream side (right-hand side). Such a filter is called generally "cross flow system."